1. Field of the Invention
The present invention relates generally to an HSDPA communication system, and in particular, to an apparatus and method for reporting a channel quality of a downlink to a Node B by a UE in an HSDPA service.
2. Description of the Related Art
In general, HSDPA (High Speed Downlink Packet Access) refers to a technique for transmitting data using HS-DSCH (High Speed-Downlink Shared Channel), a downlink data channel for supporting high-speed downlink packet transmission, and its associated control channels in a UMTS (Universal Mobile Telecommunication System) communication system. AMC (Adaptive Modulation and Coding), HARQ (Hybrid Automatic Retransmission Request), and FCS (Fast Cell Selection) techniques have been proposed in order to support the HSDPA. The AMC, HARQ and FCS techniques will be described herein below.
First, the AMC will be described. The AMC is a data transmission technique for adaptively determining a modulation technique and a coding technique of a data channel according to a channel condition between a Node B and a UE (User Equipment), thus to increase the overall utilization efficiency of the Node B. Therefore, the AMC supports a plurality of modulation techniques and coding techniques, and modulates and codes a data channel signal by combining the modulation techniques and the coding techniques. Commonly, each combination of the modulation techniques and the coding techniques is called “MCS (Modulation and Coding Scheme),” and there are defined a plurality of MCS levels of #1 to #n according to the number of the MCSs. That is, the AMC adaptively determines an MCS level according to a channel condition of a UE and a Node B to which the UE is wirelessly connected, thereby increasing the entire utilization efficiency of the Node B.
Next, the HARQ will be described, especially n-channel SAW HARQ (Stop and Wait Hybrid Automatic Retransmission Request). The HARQ newly proposes the following two schemes in order to increase transmission efficiency of the existing ARQ (Automatic Retransmission Request). First, a retransmission request and a response of the request are exchanged between a UE and a Node B. Second, defective data is temporarily stored, and combined with retransmitted data corresponding to the defective data. Further, the HSDPA has introduced the n-channel SAW HARQ in order to make up for a shortcoming of the conventional SAW ARQ. The SAW ARQ does not transmit the next packet data until it receives ACK for the previous packet data. Therefore, in some cases, the SAW ARQ must await ACK, although it can currently transmit the next packet data. However, in the n-channel SAW HARQ, the next packet data is continuously transmitted before ACK for the previous packet data is received, thereby increasing utilization efficiency of channels. That is, if n logical channels are established between a UE and a Node B, and the n logical channels can be identified by time and unique channel numbers, then the UE can recognize a channel over which packet data was received, and rearrange the received packets in the right reception order, or soft-combine the received packets.
Finally, the FCS will be described. The FCS is a technique for fast selecting a cell having the best channel condition among a plurality of cells, when a UE supporting the HSDPA (hereinafter referred to as “HSDPA UE”) is located in a cell overlapping region, or a soft handover region. Specifically, in the FCS, if an HSDPA UE enters a cell overlapping region between an old Node B and a new Node B, the UE establishes radio links to a plurality of cells, i.e., the old Node B and the new Node B. Here, a set of the cells to which the UE has established the radio links is called an “active set.” The UE reduces overall interference by receiving HSDPA packet data only from the cell maintaining the best channel condition among the cells included in the active set. Here, a cell in the active set, which transmits HSDPA packet data due to its best channel condition, is called a “best cell,” and the UE periodically checks channel conditions of the cells in the active set and transmits a best cell indicator to the cells belonging to the active set in order to replace the current best cell with a new best cell having the better channel condition. The best cell indicator includes a cell ID of a cell selected as the best cell, and the cells in the active set receive the best cell indicator and detect the cell ID included in the best cell indicator. Each of the cells in the active set determines whether the received best cell indicator includes its own cell ID. As a result of the determination, if the best cell indicator includes its own cell ID, the corresponding cell transmits packet data to the UE over HS-DSCH.
FIG. 1 schematically illustrates a downlink channel structure in a general HSDPA communication system. Referring to FIG. 1, a downlink dedicated physical channel (DPCH) includes fields defined in the exiting CDMA (Code Division Multiple Access) communication system, e.g., a Release-99 system, and an HS-DSCH indicator (HI) indicating whether there is HSDPA data to be received at a UE. The HI transmitted over the downlink DPCH not only informs whether there is HSDPA data to be received at a corresponding UE, but also informs a channelization code of a shared control channel (SHCCH) that should transmit control information for HS-DSCH over which the HSDPA packet data is actually transmitted. If necessary, a part of HS-DSCH control information, e.g., such control information as an MCS level may be transmitted over the HI.
For example, in the case where the HSDPA packet data is transmitted in a unit of N (=N1+N2) slots (i.e., HSDPA Transmission Time Interval (TTI)=N slots), if a slot format is fixed in the TTI, the HI is separately transmitted at N1 slots and a part for transmitting the HI at the remaining N2 slots is transmitted at N2 slots on a DTX (Discontinuous Transmission) basis. For example, in FIG. 1, when the HSDPA packet data is transmitted in 3 slots unit (i.e., 1 TTI=3 slots), the HI is transmitted at one of the 3 slots. Now, reference will be made to the remaining two slots where the HI is not transmitted. A slot in one TTI, for transmitting the HI, must have an HI field. Therefore, if there exists packet data that a UE must receive, corresponding HI bits are inserted in the HI field before being transmitted. In contrast, if there exists no packet data that the UE must receive, the HI field undergoes DTX. However, the other slots in the TTI, which are not required to transmit the HI, i.e., the remaining two slots can be managed in the following two methods. The two slots in the TTI, which are not required to transmit the HI, may not have an HI field in slot structure. If the remaining two slots not transmitting the HI have the HI fields, the HI fields will undergo DTX, since they do not transmit the HI. However, if a Node B previously recognizes the slots which do not transmit the HI, it is not necessary to assign the HI fields to the corresponding slots. Therefore, the remaining two slots not transmitting the HI do not have the HI fields, and instead, have the same slot format as the DPCH slot format of the existing non-HSDPA communication system, e.g., Release-99 system. That is, a first method is to fix a slot format in the TTI and assign an HI field to each slot in the TTI, while a second method is to adaptively control the slot format in the TTI.
Here, HS-DSCH control information is transmitted over SHCCH, and the HS-DSCH control information transmitted over the SHCCH includes:
(1) Transport Format and Resource related Information (TFRI): this indicates MCS level, HS-DSCH channelization code information and transport format information, all to be used for HS-DSCH.
(2) HARQ information
(a) HARQ processor number: in n-channel SAW HARQ, this indicates a channel to which specific packet data belongs among logical channels for HARQ.
(b) HARQ packet number: in the FCS, if a best cell is changed, a unique number of downlink packet data is informed to a UE so that the UE can inform a selected new best cell of a transmission state of the HSDPA data.
(3) CRC (Cyclic Redundancy Check)
CRC is generated from the TFRI, HARQ information, and UE ID (Identity). The UE ID servers as an identifier of a UE which must receive corresponding SHCCH. Therefore, the CRC is used not only to detect an error in the SHCCH but also to determine whether a UE has decoded SHCCH assigned to the UE itself.
Further, the SHCCH can be assigned one or more channelization codes. For example, in FIG. 1, the number of SHCCHs that can be assigned to UEs is a maximum of 4. Therefore, HI of the DPCH not only informs whether there exists HSDPA packet data to be received, and but also informs information on SHCCH that a corresponding UE must receive. Since the number of assignable SHCCHs is a maximum of 4, a 2-bit HI can be used to indicate information on the SHCCH that the UE must receive. For example, the UE receives SHCCH#1 for HI=00, SHCCH#2 for HI=01, SHCCH#3 for HI=10, and SHCCH#4 for HI=11. In addition, the HS-DSCH is a channel over which HSDPA packet data is transmitted from the Node B to the UE.
Now, a description will be made as to a process of receiving an HSDPA service by a UE using the above-stated channels DPCH, SHCCH and HS-DSCH.
First, the UE receives a downlink DPCH signal and decodes bits transmitted by an HI field. If the HI field undergoes DTX, the UE recognizes that there exists no HSDPA packet data to receive, and waits until the next TTI while receiving only a DPCH signal. However, if a specific bit value is transmitted as the HI, the UE recognizes that there exists HSDPA data to receive, and receives a corresponding SHCCH signal according to the HI bit value. Thereafter, the UE reads the corresponding SHCCH signal, and extracts MCS level, channelization code and HARQ-related control information for HS-DSCH, needed to demodulate an HS-DSCH signal. Finally, the UE receives the HS-DSCH signal using the control information detected through the SHCCH, and detects HSDPA packet data by demodulating the received HS-DSCH signal.
As stated above, in order to demodulate an HS-DSCH signal, the UE must read an SHCCH signal and detect corresponding control information from the SHCCH. That is, as illustrated in FIG. 1, the UE must receive the HS-DSCH signal after first receiving DPCH and SHCCH signals and reading control information from the received signals. That is, a start point of the downlink DPCH goes ahead of start points of the SHCCH and HS-DSCH. This is because before reading the HI and detecting the corresponding information, the UE cannot recognize whether the remaining two channels include data corresponding to the UE. That is, before reading the HI, the UE cannot recognize whether the received data corresponds to the UE, so the data should be temporarily stored in a buffer. Therefore, the UE receives the remaining two channels after allowing time to read the HI, thereby to reduce a load on the buffer. As a result, the UE determines whether there exists HSDPA packet data to receive by reading the HI part in the downlink DPCH, and reads HS-DSCH control information of the SHCCH if there exists HSDPA packet data to receive. Based on the control information, the UE receives the HSDPA packet data over the HS-DSCH.
Next, an uplink channel structure of a general HSDPA communication system will be described with reference to FIG. 2.
FIG. 2 schematically illustrates an uplink dedicated physical channel in a general HSDPA communication system. In the HSDPA communication system, UEs can assign all available OVSF (Orthogonal Variable Spreading Factor) codes for transmission of an uplink dedicated physical channel (DPCH), so channelization code resources are sufficient. If an uplink control channel's slot format of the non-HSDPA communication system, e.g., Release-99 communication system, is modified, a compatibility with the HSDPA communication system is not maintained and the uplink channel structure may be complicated. Therefore, an uplink control channel for the HSDPA communication system has been defined using a new channelization code. The uplink dedicated physical control channel (DPCCH) defined using a new channelization code can be used in both the HSDPA communication system and the non-HSDPA communication system. In this case, even when an HSDPA UE communicates with a Release-99 Node B, it is not necessary to modify the DPCCH slot format. Herein, DPCCH for the HSDPA communication system will be called “HS-DPCCH.”
Referring to FIG. 2, the uplink DPCH includes the DPCH structure defined in the existing non-HSDPA communication system, e.g., Release-99 communication system. A description will be made of an uplink dedicated physical data channel (DPDCH) and an uplink DPCCH of the uplink DPCH. The DPDCH slots transmit upper layer data from a UE to a Node B. Each slot of the DPCCH includes Pilot field, TFCI (Transport Format Combination Indicator) field, FBI (FeedBack Information) field, and TPC (Transmit Power Control) field. The Pilot field transmits a pilot symbol, and the pilot symbol is used as a channel estimation signal when the UE modulates data to be transmitted to the Node B. The TFCI field transmits TFCI bits, and the TFCI bits indicate TFC (Transport Format Combination) used by the currently transmitted data. The FBI field transmits a feedback information symbol, and the feedback information symbol is transmitted when transmission diversity is used. The TPC field transmits a TPC symbol, and the TPC symbol is used to control transmission power of a downlink channel. In addition, the uplink DPCCH is spread with an OVSF code before being transmitted, and a spreading factor (SF) currently used is fixed at 256.
In addition, control information that must be transmitted over an uplink in order to support the HSDPA, lies in the following two types of control information.
A first type of the control information consists in an ACK (Acknowledgement) signal or a NACK (Negative Acknowledgement) signal. In the HSDPA communication system, upon receiving data transmitted by a Node B, a UE checks whether the received data has an error, and transmits the ACK or NACK according to the error check result. In the SAW ARQ, the ACK or NACK can be expressed with 1 bit. Likewise, in the HSDPA where the n-channel SAW ARQ is used, the ACK or NACK is assigned only 1 bit.
A second type of the control information consists in a channel quality indicator (CQI).
Upon receiving a downlink channel signal, a UE measures a channel quality of the received downlink channel signal and reports the measured channel quality to a Node B. The Node B receives the channel quality information, determines an MCS level of HS-DSCH according to the channel quality, and generates TFRI, HS-DSCH control information. For example, as a result of analyzing the channel quality information reported by the UE, if the channel condition is good, the Node B selects a modulation technique of 16QAM (16-ary Quadrature Amplitude Modulation) which can increase a data rate at the sacrifice of a bit error rate (BER). In contrast, if the channel condition is poor, the Node B selects a modulation technique of QPSK (Quadrature Phase Shift Keying) having a lower BER.
The ACK/NACK and the CQI are transmitted over the HS-DPCCH. For example, in HS-DPCCH having a 3-slot TTI structure, one slot transmits the ACK/NACK and the remaining two slots transmit the CQI. Although the ACK/NACK is transmitted over a first slot of the TTI in FIG. 2, the slot transmitting the ACK/NACK can be changed. A spreading factor of a channelization code used for the HS-DPCCH in order to support the HSDPA is 256, like the spreading factor for the DPCCH in the Release-99 system. That is, the DPCCH uses a first OVSF code among OVSF codes with SF=256, whereas the HS-DPCCH uses a different OVSF code from the OVSF code used for the DPCCH. If the spreading factor of the HS-DPCCH is fixed at 256, the number of bits transmitted over one slot is fixed to 10. As a result, the number of ACK/NACK bits becomes 10, and the number of CQI bits becomes 20.
Next, a structure for transmitting the HS-DPCCH will be described with reference to FIG. 3.
FIG. 3 schematically illustrates a structure for transmitting an HS-DPCCH signal in a general HSDPA communication system. First, as described in conjunction with FIG. 2, in the HS-DPCCH structure, if a spreading factor is 256, ACK/NACK information is transmitted with 10 bits, and CQI information is transmitted with 20 bits. In addition, since the ACK/NACK information is expressed with 1 bit, a structure for repeating original information is necessary in order to transmit the ACK/NACK with 10 bits. Therefore, as illustrated in FIG. 3, a repeater 303 repeats 1-bit ACK/NACK 301 and outputs 10 bits. However, the CQI is transmitted with n bits. As stated above, since the CQI must be transmitted with 20 bits, the CQI should undergo channel coding in order to match the number of the CQI bits to 20. Therefore, as illustrated in FIG. 3, a channel coder 304 channel-codes n-bit CQI 302 at a preset coding rate, e.g., a coding rate of (20,n), and generates 20 coded bits. The generated 10-bit ACK/NACK and 20-bit CQI are inserted into corresponding slots according to a switching operation by a switch 307. When there is no ACK/NACK information or CQI information to be transmitted to the Node B, the UE performs a DTX operation.
Now, a method for generating CQI information by the UE according to the quality of a downlink channel signal will be described.
The CQI information is used by a Node B to determine an MCS level of HS-DSCH. The Node B uses a high data rate, or high MCS level, if a downlink channel has a good channel condition. Otherwise, if the downlink channel has a bad channel condition, the Node B uses a low data rate, or low MCS level. The Node B transmits the HS-DSCH in the determined MCS level. Commonly, the channel quality can be determined through a measured carrier-to-interference ratio (C/I) of a common pilot channel (CPICH). However, when the UE simply transmits only the channel condition to the Node B, condition variety of the UE is accommodated. That is, even though the channel condition is constant, the Node B may support a higher MCS level when the UE has higher performance. However, since the Node B cannot recognize performance of the UE, the Node B will determine an acceptable MCS level on the basis of a UE having normal performance. Therefore, it is preferable for the UE to generate CQI information taking its performance into consideration.
In addition, as stated above, a Node B determines an MCS level of HS-DSCH according to the CQI information received from a UE. If the Node B one-sidedly determines an MCS level for HS-DSCH, it is not possible to take the variety of performance of UEs into consideration. In order to determine an MCS level taking the variety of the UEs into consideration, the UEs must provide information on their performances so that the Node B can take performances of the UEs into consideration. That is, the UE checks a current channel condition by measuring C/I from CPICH, and defines the maximum acceptable transport format and TFRC (Transport Format and Resource Combination) as CQI information according to the checked channel condition, taking performance of the UE itself into consideration. The TFRC includes information on a modulation technique for HS-DSCH, a TBS (Transport Block Set) size, and the number of acceptable HS-DSCHs. Upon receiving the TFRC, for which performance of the UE was taken into consideration, from the UE, the Node B determines TFRI according to the received TFRC. The TFRI, as described in conjunction with FIG. 1, means an MCS level to be used in HS-DSCH, HS-DSCH channelization code information, and a transport format. That is, the TFRC is used by the UE to report a maximum acceptable limit to the Node B, and the Node B determines TFRI based on its capacity and the TFRC reported by the UE.
As described above, the UE determines TFRC after measuring C/I from CPICH, and examples of the TFRC that can be selected by the UE are illustrated in Table 1. Shown in Table 1 is a modulation technique for HS-DSCH, which can be accepted by the UE according to the channel condition, a TBS size, and the number of channels. For example, there may occur a case where the UE recognizes through C/I measurement that the channel condition is poor, and in addition, the UE has so low performance that it cannot process data having a high data rate. In this case, the UE selects TFRC1 and QPSK having a low BER, and sets a TBS size to 1200 to decrease a data rate, as illustrated in Table 1. In other words, the UE will select TFRC1 on the assumption that in the current channel condition, if the UE selects TFRC2, it obtains a block error rate (BLER) higher than a BLER threshold, and if the UE selects TFRC1, it obtains BLER lower than the BLER threshold. The table shown in Table 1 must be included in both the UE and the Node B. The reason is because if the UE transmits TFRC1 to the Node B, the Node B can recognize a modulation technique and a TBS size, required by the UE, by searching Table 1 for the TFRC1.
TABLE 1TFRCsModulationTBS Size# of code channelsTFRC1QPSK12005TFRC2QPSK24005TFRC3QPSK36005TFRC416QAM48005TFRC516QAM60005TFRC616QAM72005
If the UE reports TFRC to the Node B instead of the measured C/I as shown in Table 1, the Node B may fail to correctly detect the channel condition. In other words, in the case where the UE reports the measured C/I to the Node B, since the C/I is a value showing the intact channel condition, the Node B can correctly detect the channel condition. However, since the TFRC is a value indicating a modulation technique and a TBS size, a variation in the TFRC will lead to a large variation in the C/I. That is, a C/I difference between the case where the UE selects TFRC1 and the case where the UE selects TFRC2 can be extended to several dB from 1 dB. Therefore, since a C/I difference between TFRC1 and TFRC2 can become several dB, it may be impossible for the Node B to recognize the correct channel condition.
Therefore, in order to correctly report the channel condition, the UE transmits HS-DSCH power offset to the Node B. The HS-DSCH power offset is an offset value against a reference power level of HS-DSCH. Upon receiving the HS-DSCH power offset, the Node B transmits an HS-DSCH signal at transmission power determined by increasing the reference HS-DSCH power by the HS-DSCH power offset. In this manner, the UE can correctly report the quality of a downlink channel using the HS-DSCH power offset. Table 2 below shows CQI information actually transmitted to the Node B by the UE. The UE generates the CQI information as a combination of the TFRC determined from Table 1 and the HS-DSCH power offset, taking into consideration the measured C/I and its performance. In Table 2, the number of cases that can be determined by the UE is 27, so the CQI information is expressed with 5 bits. Since TFRC and HS-DSCH power offset are used in Table 2, a C/I difference between uplink signaling values becomes small. That is, when more information bits are used, a C/I difference between uplink signaling values may become 1 dB, and when less information bits are used, the C/I difference between the uplink signaling values may become higher than 1 dB.
TABLE 2TFRCPower offsetUL signaling valueTFRC112 dB 011 dB 110 dB 29 dB38 dB47 dB56 dB65 dB74 dB83 dB92 dB101 dB110 dB12TFRC22 dB131 dB140 dB15TFRC32 dB161 dB170 dB18TFRC42 dB191 dB200 dB21TFRC52 dB221 dB230 dB24TFRC62 dB251 dB260 dB27
The UE periodically reports the determined CQI information of Table 2 to the Node B. However, when the UE is located in a soft handover (SHO) region, a condition of the downlink channel is poor, and in addition, a TPC command transmitted over an uplink DPCCH fails to correctly take a condition of the downlink channel into consideration. On the other hand, when the UE is not located in the SHO region, a condition of the downlink channel is better than when the UE is located in the SHO region, so the UE reports the CQI information at longer periods.
When the UE is located in the SHO region, an HS-DSCH signal is received from only one cell. However, unlike the HS-DSCH signal, downlink DPCH signals are received from a plurality of cells, i.e., the cells in the active set. Therefore, the UE soft-combines the downlink DPCH signals received from the cells, and then, measures a condition of the downlink DPCH. Based on the measured result, the UE transmits a TPC command for transmission power control on the downlink channel to the Node B. The TPC command includes information on transmission power for which the soft combining was taken into consideration. That is, the UE takes into consideration not only a condition of the HS-DSCH received from only one cell, but also a condition of the soft-combined downlink DPCH.
That is, the UE, when it is located in the SHO region, transmits CQI information to the Node B every TTI in order to correctly report a condition of the HS-DSCH. However, if all UEs located in the SHO region report the CQI information to the Node B every TTI, considerable uplink interference occurs. Further consideration should be taken into the fact that in an actual channel environment, an average channel condition is not abruptly changed every TTI. Accordingly, there have been demands for a method of correctly reporting the downlink channel quality while minimizing uplink interference.